FLOW AROUND A CYLINDER - istiarto

3.5 Summary and conclusions
– 3.34 –
The measured 3D instantaneous velocity profiles presented in the preceding chapter (see
Chapter 2) were analyzed. The emphasize of the analyses is put on the (time-averaged)
velocity fields, notably their spatial variation in vertical planes around the cylinder, in
order to investigate the flow alteration due to the existence of the cylinder and the scour
hole. The turbulence characteristics, such as the turbulence intensities, the turbulent
kinetic energy, and the Reynolds stresses, were also analyzed. An attempt was also made
to estimate the magnitude of the bed shear-stresses in the scour hole along the plane of
symmetry.
A 3D flow establishes around the cylinder, being characterized principally by a clockwise
circulating flow inside the scour hole (see Fig. 3.2). This structure, known as a
horseshoe vortex, was detected particularly in the plane of symmetry upstream of the
cylinder. Moving around the cylinder towards downstream, it diminishes and becomes
practically undetected on the side plane. Downstream of the cylinder, a flow reversal
towards the surface was observed, being pronounced in the close vicinity of the cylinder.
It gradually disappears as the flow moves away from the cylinder and returns back
towards the uni-directional flow condition.
The velocity field outside the scour hole, i.e. in the upper layer above the original bed, is
dominated by the longitudinal velocity component; only at the vicinity of the cylinder the
transverse and, notably, the vertical velocity components are important (see Fig. 3.1). The
flow direction of the approach flow passing the cylinder remains much the same. The
effect of the cylinder in deflecting the approach flow is limited to regions close to the
cylinder and in the scour hole.
The vertical velocity component, which primarily manifests itself as a downward velocity
particularly along the cylinder face, was separately investigated. Its spatial variation
around the cylinder was found to have a particular characteristic. Along the cylinder face
its maximum values fall at 40% of the local flow depth (see Fig. 3.6) irrespective of the
angular direction.
Presented in Fig. 3.13 are the measured flow pattern, showing the velocity components,
(ur,w), in different planes around the cylinder, whereas in Fig. 3.14 are the corresponding
main velocity components, (u,w), in those planes. It can be clearly seen the unidirectional
flow experiencing an alteration due to the cylinder and the scour hole,
becoming a three-dimensional one. This alteration is notably observed in the region close
to the cylinder and inside the scour hole. Outside the scour hole, the flow pattern is
mainly dominated by the longitudinal velocity components.
The intensity of turbulence inside the scour hole is strong; an increasing turbulence was
detected approaching the cylinder and moving around the cylinder towards downstream.
In the wake region, where a separation evidenced by a flow reversal takes place, the
turbulence attains its strongest intensity (see Fig. 3.9). The kinetic energy of the flow
inside the scour hole, where a rotating flow is eminent, consists of high turbulent energy